Q.(a) What are boranes? Draw the structure of diborane.

Diborane's structure has 4 terminal B-H bonds (normal) and 2 bridging B-H-B bonds, each bridge being an unusual 3-centre-2-electron bond, because boron does not have enough valence electrons for a normal Lewis structure.

Boron has only 3 valence electrons, so BH3 alone would be electron deficient (only 6 electrons around B, not the octet of 8). Two BH3 units dimerise to form B2H6 to achieve extra stability. In diborane:

• Each boron atom is bonded to 2 terminal hydrogen atoms by ordinary, 2-centre-2-electron (2c-2e) covalent B-H bonds. These 4 terminal B-H bonds all lie in one plane.
• The two boron atoms are additionally bridged by 2 hydrogen atoms, one lying above and one below the plane of the terminal hydrogens. Each B-H-B bridge is held together by only 2 electrons shared among 3 atoms (1 boron - 1 hydrogen - 1 boron) — this is called a 3-centre-2-electron (3c-2e) bond, sometimes nicknamed a 'banana bond' because of its curved electron-density shape.
• Each boron atom is sp3 hybridised, using its four sp3 hybrid orbitals for the 2 terminal B-H bonds and the 2 bridge bonds (each bridge bond uses one sp3 orbital from each boron plus the hydrogen 1s orbital). …